Such a technique is known in relation to an internal combustion engine including a three-way catalyst, a NOX storage reduction (NSR) catalyst, and an SCR catalyst which are arranged in an exhaust passage that the air-fuel ratio of an air-fuel mixture subjected to the combustion in the internal combustion engine is set to a weak lean air-fuel ratio (slight lean air-fuel ratio) if there is a possibility that NH3, which is adsorbed to the SCR catalyst, may be discharged or emitted into the atmospheric air, and thus NOX emitted from the internal combustion engine and NOX desorbed from the NSR catalyst are allowed to arrive at the SCR catalyst so that NH3, which is adsorbed to the SCR catalyst, is consumed (see, for example, Patent Literature 1).
Further, Patent Literature 2 discloses such a technique in relation to an internal combustion engine having an exhaust passage arranged with an SCR catalyst and an injection nozzle for injecting urea or NH3 into the exhaust gas upstream from the SCR catalyst that when the temperature of the SCR catalyst is suddenly raised, then the injection of urea or NH3 from the injection nozzle is stopped, and the amount of NOX emitted from the internal combustion engine is increased so that NH3 desorbed from the SCR catalyst is thereby reacted with NOX.